1. Field of the Invention
The invention relates generally to the field of marine seismic survey systems. More particularly, the invention relates to apparatus for determining strain in lead-in cables used with marine seismic survey systems and methods for using strain measurements from such cables.
2. Background Art
Marine seismic surveying systems are used to acquire seismic data from Earth formations below the bottom of a body of water, such as a lake or the ocean. Marine seismic surveying systems typically include a seismic vessel having onboard navigation, seismic energy source control, and data recording equipment. The seismic vessel is typically configured to tow one or more streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals produced by various sensors on the one or more streamers are ultimately conducted to the recording equipment, where a record with respect to time is made of the signals produced by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the Earth formations below the bottom of the body of water.
The one or more streamers are in the most general sense long cables that have seismic sensors disposed at spaced apart positions along the length of the cable. A typical streamer can extend behind the seismic vessel for several kilometers. Because of the great length of the typical streamer, movement in the water produces a great deal of drag (friction). More recently, marine seismic acquisition systems have been designed that include a plurality of such streamers towed by the seismic vessel in parallel. The streamers are towed by the vessel using towing devices, including cables called “lead-in” cables, and associated equipment that maintain the streamers at selected lateral distances from each other as they are towed through the water.
In combination, the drag produced by the plurality of streamers is quite large, causing the lead-in cables and related towing equipment to undergo considerable stress during survey operations. It is important, for purposes of maintaining the selected lateral distance between the streamers and to reduce the possibility of excessively stressing any one or more of the cables in the towing equipment, among other purposes, to properly distribute the strain on the various lead-in cables. Accordingly there is needed a system for measuring strain in the lead-in cable in order that the strain may be properly distributed among the various lead-in cables.